The present invention relates to a method for manufacturing a Nb.sub.3 Sn superconductor as well as a method for manufacturing a hollow superconducting magnet.
As well known, intermetallic compound type superconducting material, particularly Nb.sub.3 Sn type superconducting material is expected to be available for a superconductor of superconducting magnet which is intended for nuclear fusion reactor or the like, because said superconducting material has remarkable superconductivity in comparison with other compound type or alloy type superconducting materials. Due to high brittleness and low workability, particularly low malleability and ductility of Nb.sub.3 Sn, however, it is difficult to work the Nb.sub.3 Sn material in a form of rod or pipe to the predetermined reduced diameter by means of usual plastic working. In view of the aforesaid drawbacks with the Nb.sub.3 Sn intermetallic compound a modified method for manufacturing Nb.sub.3 Sn type superconductor was proposed, which comprises the steps of subjecting a composite conductor element in a form of rod or the like to a diameter reducing working until the required diameter is reached, said composite conductor element containing Nb and Sn still in a metallic state and then heating up the diameter reduced composite conductor element to the predetermined temperature so that Sn is diffused to form an intermetallic compound of Nb.sub.3 Sn.
It is recognized with the above known method that pure Nb may contact directly with pure Sn in the form of composite conductor element of Nb wire and Sn wire extending longitudinally side by side and the composite conductor element is subjected to diffusion heat treatment at an elevated temperature in the aforesaid arrangement. Alternatively, Cu may be employed as an Sn diffusion carrier so that the latter in diffused through the former at a lower temperature with much more formation of Nb.sub.3 Sn than in case of the aforesaid direct diffusion. In practice, therefore, Cu is preferably employed as an Sn-diffusion carrier. As typical embodiments of employing Cu as an Sn-diffusion carrier the following methods were proposed: One of them is a so-called bronze method in which one or more pure Nb cores are inserted in Cu-Sn alloy matrix (bronze) in a form of rod, the Nb-inserted assembly is subjected to a diameter reducing working and then the diameter reduced assembly is heated up so as to form an intermetallic compound layer of Nb.sub.3 Sn around the Nb cores. The other one is a so-called Sn-plating method in which one or more pure Nb cores are inserted in a pure copper rod, the Nb-inserted rod is subjected to a diameter reducing working, Sn-plating is formed on the outer surface of the diameter reduced rod and then the Sn-plated rod is heated up to form an intermetallic compound layer of Nb.sub.3 Sn around the Nb cores. In fact, however, these conventional methods have advantages and disadvantages respectively and thus they are far from satisfactory.
Specifically the former bronze method has an advantage in that a sufficient amount of Nb.sub.3 Sn is formed in a for relatively short period of time and moreover there is no requirement for controlling the thickness of the Sn-plating layer, but it has a disadvantage of less workability during the diameter reducing operation. Further the bronze method has another disadvantage in that usually a Cu-Sn alloy containing Sn 10 to 14 weight percent must be employed for practicing this method, therefore this Cu-Sn alloy has a tendency of work-hardening during the diameter reducing operation, which usually requires annealing to be effected when the reduction rate of sectional area reaches about 75 percent. In case each Nb core has to be reduced to several microns in diameter as is the case with multicore superconductors, annealing is repeatedly carried out, resulting in remarkably increased work hours and extremely reduced work efficiency. To eliminate the aforesaid disadvantage an improved bronze method was proposed, in which the Cu-Sn alloy matrix is sheathed with a Cu pipe having excellent workability and then the Cu-sheathed assembly is subjected to a diameter reducing working. It is pointed out as a problem with the improved method that Sn in the Cu-Sn alloy matrix is diffused into the outer Cu pipe during diffusion heat treatment, causing a shortage in Sn to take place, whereby a sufficient amount of Nb.sub.3 Sn is formed only with much difficulty.
On the other hand, in the latter Sn-plating method, copper having good workability is sheathed over the Cu-Sn matrix for practicing this method, which causes annealing to be carried out in substantially less times than in case of the former method, but it is required to form Sn-plating having a thickness more than that required to form Nb.sub.3 Sn on the outer surface of the Cu sheath after completion of the diameter reducing operation. Moreover it takes a long time to form such a thick Sn-plating and the required thickness is difficult to control. Further it is pointed out as another drawback with the Sn-plating method that since Sn is supplied from the Sn-plating layer apart from the Nb cores so as to form intermetallic compound of Nb.sub.3 Sn, the efficiency is lower than that of the former bronze method and moreover it takes long time to form a sufficient amount of Nb.sub.3 Sn.
As is well known, hollow superconductor having a longitudinal passage through which a cooling medium such as He or the like flows is preferably used for constructing a superconducting magnet. A typical known method for manufacturing the above type of superconductor is such that superconductor element is spirally wound around a hollow pipe made of metallic material having good conductivity such as copper or the like. This conventional method is suitably applied for manufacturing alloy-type superconductors having comparatively excellent workability, for instance, NbTi type superconductors. A drawback of the aforesaid conventional method is the difficulty of manufacturing intermetallic compound type superconductors, for instance, Nb.sub.3 Sn, V.sub.3 Ga, Nb.sub.3 Ge type superconductors. Namely in case of an intermetallic compound type superconductor its, characteristics are worsened due to bending stress which is caused during the operation of winding an intermetallic compound superconductor element around the hollow copper pipe mainly because of high brittleness and reduced workability, particularly reduced malleability and ductility. In an extreme case the winding operation itself is difficult to be carried out. In view of the drawbacks with the known methods as mentioned above, the inventors invented a method for manufacturing an intermetallic compound type superconductor without a winding operation, as disclosed in Japanese Patent Application Nos. 131264/77 and 131265/77 corresponding to U.S. Pat. No. 4,151,062. This method is such that there is provided a hollow conductor carrier made of good conducting material such as copper in a rectangular cross-sectional shape, which has a longitudinal passage in the interior thereof, through which a cooling medium flows, and has grooves on the four sides thereof, while a plurality of conductor elements (in a metallic state where no intermetallic compound appears) with which the intermetallic compound type superconductor is constructed are braided and the braided conductor element assembly is worked to such a shape as to fit into said grooves, then the preformed conductor element assembly is subjected to diffusion heat treatment so that an intermetallic compound is formed and thereafter the diffused multicore conductor element is secured to said grooves with the aid of a low temperature melting metal such as soft solder. It has been recognized as an advantage with this method that since the conductor element assembly is worked to such a shape as to fit into the groove of the hollow conductor carrier prior to forming an intermetallic compound, no additional working is required after formation of the intermetallic compound, which means that there is no danger of worsening or deteriorating the characteristics of the obtained superconductor.
In manufacturing a superconducting magnet with the use of the hollow superconductor provided in accordance with the method as proposed above, usually the superconductor assembly is wound around a hollow magnet reel, after the respective superconductor is secured to the groove of the hollow conductor carrier. During the aforesaid winding operation compressive strain generates on the multicore superconductor located on the inner side toward the magnet reel, while tensile strain generates on the multicore superconductor located on the outer side apart from said magnet reel. When the winding diameter is very large relative to the diameter of the magnet reel or, conversely, when the superconductor has a small diameter relative to the winding diameter, said compressive and tensile strains are small respectively, whereby the multicore superconductor is scarcely injured and the characteristics thereof are little deteriorated. On the contrary, however, when the winding diameter is small or the diameter of the superconductor is very large, there is a danger that the multicore superconductor is injured or damaged and the characteristics thereof are worsened or deteriorated. In the worst case the outer multicore superconductor may be broken due to tensile strain, while the inner multicore superconductor may be buckled due to compressive strain.